Method for locating and removing manhole covers

ABSTRACT

The presence of a manhole cover in a roadbed under a layer of non-magnetic asphalt is detected and accurately located by a method in which magnetic detection apparatus have a detection alarm and employing inductive search coils is operative to sense the proximity of a metallic object, at least portions of which are magnetically detectable, through changes in coil inductance. The detectiom method and apparatus is capable of determining the thickness of the asphalt layer and heat is employed to efficiently melt through the asphalt in such a way as to permit removal of the cover without unnecessarily damaging the asphalt surface. Thereafter, the cover may be replaced in the road bed and resealed in place.

United States Patent 11 1 Lucero et al. Oct. 9, 1973 [5 METHOD FORLOCATING AND 3,355,658 11/1967 Gardiner 324 3 REMOVING MANHOLE COVERS v2,931,383 4/1960 Handley 324/3 X [75 Inventors: Frank J. Lucero; Jack A.Lucero, FOREIGN PATENTS OR APPLICATIONS both of Denver; Gary R. Weekly,508,347 9/1930 Germany 324/67 Littleton, all of Colo.

[73] Assignee: Lee Way Trenching, Inc., Denver,

[22] Filed: Feb. 18, 1971 [21] App]. No.: 116,567

[52] US. Cl. 404/79, 324/3 [51] Int. Cl. E01c 23/14 [58] Field of Search94/34, 22; 137/369; 324/3, 41, 67

[56] References Cited UNITED STATES PATENTS 2,856,580 10/1958 Clark324/67 X 3,492,564 l/l970 Baker 324/41 X 1,950,169 3/1934 Farasay 94/1.5

3,250,190 5/1966 Taylor 94/34 3,601,691 8/1971 Gardiner 324/3 3,549,98512/1970 Penland 324/3 Primary Examiner-Nile C. Byers, Jr. Att0rneyRei1ly& Lewis [57] ABSTRACT The presence of a manhole cover in a roadbed undera layer of non-magnetic asphalt is detected and accurately located by amethod in which magnetic detection apparatus have a detection alarm andemploying inductive search coils is operative to sense the proximity ofa metallic object, at least portions of which are magneticallydetectable, through changes in coil inductance. The detectiom method andapparatus is capable of determining the thickness of the asphalt layerand heat is employed to efficiently melt through the asphalt in such away as to permit removal of the cover without unnecessarily damaging theasphalt surface. Thereafter, the cover may be replaced in the road bedand resealed in place.

7 Claims, 6 Drawing Figures PAIENTED 91975 3.764.223

sum 10F 3 v 24 FIG. 2

INVENTORS I FRANK J. LUCERO JACK A. LUCERO FIG. 3 GARY R. WE EKLYJaw/1w? PATENIEUucI 91915 SHEET 2 0F 3 RHEOSTAT FIG. 5

PATENTED 973 SHEET 3 or 3 hmm z METHOD FOR LOCATING AND REMOVING MANHOLECOVERS The present invention relates to locating metallic objects andmore particularly to a novel method of accurately determining thelocation and dimension of an imbedded object, such as, a metallicmanhole cover covered over by asphalt, removing same, and replacing it,and also to novel apparatus for accurately locating the object.

Manholes are customarily imbedded in roads or streets to provide a meansof access to the underground passageways formed beneath the streets.Such underground passages are used to contain gas lines, water mains,conduct sewage, etc. These manholes when not in use are generally closedby metallic manhole covers or lids of standardized shape and dimension.

In the maintenance of roads or streets, it is frequently necessary tocoat the street with a smooth layer of asphalt or other road surfacingmaterial in order to provide a smooth roadbed for vehicular travel. Toavoid uneven areas in the asphalt coating and facilitate itsapplication, the layer of asphalt is generally laid directly over themanhole. From time to time, it is necessary to reopen the concealed andimbedded manhole covers, for example, to check for the presence ofexplosive gas in the underground passageways or to check and repair thegas lines, water mains, etc., contained therein. The problem thereforeis created of relocating the concealed manhole covers in a safe,accurate manner and of removing these covers to open the manholeswithout unnecessarily damaging the smooth asphalt surface covering theroadbed.

It is accordingly an object of the present invention to provide a novelmethod and apparatus for accurately locating the outer periphery of animbedded object, such as, a manhole cover.

It is further an object of the present invention to provide a novel andimproved method for accurately locating a concealed object, such as, amanhole cover, characterized by employing magnetic detector means.

It is additionally an object of the present invention to provide a novelmethod and means for locating a concealed magnetizable object, such as,an iron manhole cover, characterized by employing a relatively largeinductive search coil or loop with electrical means to generally locatethe magnetic object and employing a relatively small inductive searchcoil or loop with the same electrical means to accurately locate theouter periphery of the magnetic object.

It is also an object of the present invention to provide a novel methodfor removing a located object, such as, a manhole cover, covered byasphalt or the like characterized by melting through the asphalt aroundthe periphery of the object so as to permit its removal.

It is another object of the present invention to provide a novel methodfor accurately locating an imbedded metallic object, such as, an ironmanhole cover, determining the depth below surface at which it islocated, as well as for removing same and replacing it.

It is still another object of the present invention to provide a novelmethod for locating, removing and replacing a magnetizable object, suchas a manhole cover, covered over by asphalt or the like characterized bythe steps of detecting the general location of the object, accuratelylocating the outer periphery of the object, determining the depth of theobject below surface, melting through the asphalt around the peripheryof the object so as to permit its removal without unnecessarily damagingthe asphalt surface, and replacing the manhole cover in place once thesubsurface operations have been performed.

It is still a further object of the present invention to provide a noveldetecting apparatus for accurately locating the periphery of a magneticobject covered over by a non-magnetic layer of material.

In accomplishing the above and other objects, there is provided a novelmethod for accurately locating, exposing, removing and replacing an ironmanhole cover in a roadbed which is covered over and concealed by asmooth continuous layer of asphalt. The manhole cover is initiallylocated by use of a relatively large inductive search coil or looparound which current is driven by an electrical circuit. The electricalcircuit includes alert means which are operated by the circuitry toindicate the presence of a magnetic object detected by a change in theloop inductance thereby to generally locate the manhole cover. Arelatively small inductive loop or search coil is then used with thesame electrical circuit to accurately locate the periphery of the coverand to determine the depth of the cover below the asphalt surface. Aheating element is placed around the located periphery of the manholecover and with the aid of an asbestos blanket the asphalt layer aroundthe periphery of the cover is melted through. The'cover is next removedafter which the manhole is inspected and/or entered for the purposedesired. Thereafter the cover is replaced and the asphalt layer isrepaired so that a smooth roadbed is formed.

The above and other objects, advantages and features of the presentinvention will become more readily appreciated and understood from aconsideration of the following detailed description when taken with theaccompanying drawings, in which:

FIG. 1 is a plan view of a relatively large search coil suitable for usein the method of present invention for detecting the presence of aconcealed manhole cover;

FIG. 2 is a plan view of a smaller search coil suitable for use in themethod of the present invention for accurately locating the periphery ofthe concealed manhole cover;

FIG. 3 illustrates the manner in which the search coil of FIG. 2 isemployed to locate in a roadbed the periphery of an iron manhole coverwhich is covered over by a layer of asphalt;

FIG. 4 illustrates the placement of a heating element covered over by anasbestos blanket for the purpose of melting through the asphalt layeraround the located periphery of the manhole cover of FIG. 3 so as topermit the removal of the manhole cover in accordance with the presentinvention;

FIG. 5 is a partial cutaway view of the asphalt layer and manhole coverof FIG. 3 illustrating the use of a pick to remove the manhole coverafter the asphalt layer around the manhole cover has been melted throughby the heating element of FIG. 4; and

FIG. 6 is a schematic diagram of the electrical circuitry making up amagnetic or metal detection apparatus according to the presentinvention.

Referring to the drawings in more detail, there is shown in FIG. 1 aninductive search coil or loop 10. The coil 10 is employed in a mannerhereinafter explained with suitable electrical circuitry to detect thepresence of a magnetic mass, such as a concealed iron manhole cover,through changes in the coils inductance. The coil is constructed to havea longitudinal dimension which is several times longer than the diameterof a standard manhole cover so as to make the coil suitable for use indetecting the general location of a manhole cover. The coil 10 is shownpositioned over a standard manhole cover 30 (shown in dashed lines) toillustrate the relative size of the coil 10 in comparison to the size ofa standard manhole cover 30. The coil 10 preferably is formed in asubstantially rectangular shape, although the coil could be formed inother suitable shapes, and has its ends 11 and 12 separated by aninsulator 13 to provide electrical isolation therebetween. Electricalleads l4 and 15 are connected to the coil ends 11 and 12, respectively,to provide means for connecting the coil 10 to suitable electricalcircuitry.

Another inductive search coil or loop designated is shown in FIG. 2. Thecoil 20 is substantially smaller than the coil 10 and is preferablycircular in shape. The coil 20 preferably has a diameter slightly lessthan the radius of a standard manhole cover and is employed withsuitable electrical circuitry in a manner hereinafter explained toaccurately locate the outer peripheral edge of a concealed magneticobject, such as an iron manhole cover. The coil 20 has ends 21 and 22which are physically separated by an insulator 23 and electricallyisolated from one another. Electrical leads 24 and 25 are connected,respectively, to the coil ends 21 and 22 to provide means for connectingthe coil 20 to suitable electrical circuitry. It is noted that the coil20 may be formed in other suitable shapes and sizes, other than thatillustrated in FIG. 2, as long as the coil 20 is suitably sized andshaped for use in accurately locating the outer edge of the concealedmagnetic object to be located through changes in the coils inductance.

FIG. 3 shows a magnetizable manhole cover 30, conventionally composed ofiron, located in a roadbed which is covered over by a layer or coatingof a nonmagnetic road surfacing material 31, such as, asphalt. The coil20 is shown in use for placing spots or marks 32, for example, byscoring the road surface, in order to accurately delineate the outerperipheral edge of the concealed manhole cover 30. The manner in whichthese spots 32 are located through use of the coil 20 is hereinafterexplained. A heating element 33 which has an AC voltage applied acrossits ends 34 and 35 is shown in FIG. 4. The heating element 33 is formedto be substantially circular in shape and is dimensioned to correspondto and fit exactly around the outer periphery of a standard manholecover 30. As shown in FIG. 4, the circular heating ring portion of theheating ele- V ment 33 is positioned symmetrically around the outer edgeof the manhole cover 30 and an asbestos blanket is placed thereover fordirecting heat from the ring 33 unidirectionally downward into theasphalt layer 31. Voltage is supplied to the heating element 33 by an ACvoltage source 36, which is preferably a 220 AC source, through arheostat 37 for controlling the voltage applied to the ends 34 and 35 ofthe heating ring 33. Electrical leads 38 and 39 are connected from theoutput of the rheostat 37 to the element ends 34 and 35, respectively.

FIG. 6 shows one suitable circuit arrangement which may be used witheither the search coil 10 or 20 for generally or accurately locating,respectively, the manhole cover 30. In the circuit 50, a search coil Lis shown connected at terminal 51 and 52, and either the inductive coil10 or 20 may be used interchangeably as the coil L. The search coil Lalong with a transistor 53, capacitor 54 and capacitor 55 function as anoscillator and the oscillator is arranged so that its maximum amplitudeof oscillation occurs when the search coil L is remotely positioned fromany magnetic mass.

With the search coil L positioned remotely from any magnetic mass,feedback capacitor 56 is adjusted until the sonar alert or alarm 57,which is preferably a type of horn, is on the verge of sounding. Thus,when the search coil L senses a magnetic object and experiences anincrease in its inductance as a function of the proximity and size ofthe magnetic mass, the sonar alert 57 will be activated by the operationof the circuit 50 to indicate that a magnetic mass has been detected.

In activating and deactivating the alarm 57, the circuit 50 functions todetect the location of a magnetic mass as follows: The transistor 53along with the circuit formed by the coil L and capacitors 54-55functions as an oscillator in which the output amplitude of itsoscillations decrease as the inductance of the coil L increases. As aresult, a negative-to-positive voltage drop appears across the resistorwhich causes the base electrode of a transistor 67 to go positive sothat the transistor 67 conducts and current flows through a sistor 68',and the output developed on the emitter electrode of the transistor 67changes the biasing on the transistor 58 so that the oscillations acrossthe transistor 53 are sustained.

The output signal developed by the transistor 59 appears on the cathodeelectrode of the diode 66. Through the action of a sensitivity networkdefined by transistor 69, resistors 70-72 and diodes 7374, the point 75becomes positive with respect to the cathode electrode of diode 76. Atthe same time, the output of a transistor which functions as anamplifier and is applied to the junction of capacitor 81, resistors82-83 and diode 84 drives a so-called Darlington transistor pairrepresented at 85. The output of the transistor pair 85 alters thepotential on the emitter electrode of transistor 86 so that a controlsignal or voltage is developed across load resistor 87 which is afunction of the position of the search coil L relative to a magneticmass. The control signal on the resistor 87 is amplified by thetransistor 80 so that a driver transistor 88 which has its baseelectrode connected to the resistor 82 drives the transistor pair 85 toconduct as a function of the search coil Ls position relative to amagnetic mass.

The amplified output of the transistor 80 is applied to the baseelectrode of a transistor 89 and this output signal may be relativelypositive or negative depending on the position of the search coil Lrelative to a magnetic mass. When the voltage applied to the baseelectrode of the transistor 89 is positive with respect to its emitterelectrode, the transistor 89 conducts so that the anode of diode 90becomes positive with respect to its cathode. Accordingly, a controlvoltage is generated across a resistor 91 and is transmitted to the baseelectrode of a transistor 93 through a resistor 92. The transistor 93has its emitter-collector path connected to shunt the sonar alert 57 sothat whenever the transistor 93 is turned on by the voltage applied toits base electrode the alert 57 is deactivated. Thus, the alert 57 isswitched on and off as a function of the position of search coil Lrelative to magnetic mass. In the operation of the magnetic detectionapparatus 50, the pitch of the sound generated by the alert 57 increasesas the search coil L gets closer to magnetic objects due to the increasein the frequency of oscillations resulting from the increase ininductance of the search coil L.

It is noted in the circuit 50 that power is supplied by a battery 100through its associated on-off switch 101 and that a transistor 102 isincluded which functions as a voltage regulator. A filter capacitor 103is connected between the emitter electrode of the transistor 93 and thebase electrode of the transistor 102. The control voltage applied to thebase electrode of the transistor 93 functions to open and close thetransistor switch 93 so as, respectively, to activate and deactivate thesonar alert 57. The transistor 104 in the circuit 50 also functions as avoltage regulator and through clamping diodes, in particular clampingdiode 105, insures that a constant bias voltage is applied to theemitter electrode of the transistor 80.

In summary, the circuitry 50 functions in conjunction with coil L (whichmay be either the inductance coil or to detect the proximity of amagnetic object by being calibrated so that when a magnetic mass is notbeing sensed by the coil L the oscillator provided by the coil L, thetransistor 53 and capacitors 54-55 oscillates at its maximum amplitudeto reverse bias the transistor 58 and the circuit 50 will function tobias the transistor 93 into conduction to shunt and deactivate the sonaralert 57. As the search coil L is brought into close proximity of amagnetic mass, the inductance of the coil L increases to approach avalue determined by the ratio of capacitors 54 and 55 so that theamplitude of the oscillators oscillations decreases towards zero. As aresult, an appropriate bias is applied to the base electrode oftransistor 58 so that the circuit 50 functions to turn on and off thetransistor switch 93 to control the activation of the alert 57 andcontrol the pitch and intensity of the alert 57 as a function of theposition of the search coil L relative to a magnetic mass.

The concealed iron manhole cover 30 of FIG. 3 which is a magnetic massand which is concealed below a non-magnetic layer of asphalt 31 may befound, accurately located, removed and replaced by the followingexemplary procedure which illustrates the preferred method of thepresent invention. The large inductive search coil 10 is first used byconnecting its electrical leads l4 and 15 to the terminals 51 and 52 ofthe electrical circuit 50. The switch 101 of the circuit 50 is closedand the coil 10 advanced across the area of the roadbed in which themanhole cover 30 is known to be located. The circuit 50 and coil 10 arecalibrated and designed for detecting a standard size iron manhole coverso that once the coil 10 is positioned substantially centrally over thecover 30 the inductance of the coil 10 increases to the predeterminedlevel necessary for the alert 57 to be energized in the mannerhereinbefore described. The position of the coil 10 at the instant thealarm energizes then indicates the general location of the cover 30. Itis noted that this general location may be ascertained more accuratelyby moving the coil in all directions until the alert 57 is switched offby the operation of the circuit 50 to indicate that the coil 10 is nolonger influenced to any significant extent by the cover 30. In thisway, the approximate area covered by the concealed cover 30 may beascertained.

The coil 10 is then disconnected from the circuitry 50 and the leads 24and of the smaller coil 20 are connected to the circuit terminals 51 and52. The small coil 20 is placed over the general location of the coverat a location at which the alert 57 is energized,

moved radially to a point that the alert 57 is deenergized, and thenmoved back towards the location of the cover 30 until the alert 57 isfirst energized. A spot 32 is then marked in the center of the coil 20,as shown in FIG. 3, such as by scoring or with paint. A plurality ofspots 32 are formed in this consistent manner thereby to form a circularpattern which is related to and outlines the location and shape of theouter edge of the cover 30. It is noted that since the thickness of theasphalt coating 31 covering the cover 30 may vary that the spots 32 maybe marked closer to the center of the cover 30 in some instances than inothers. This variation is due to the fact that the coil 20 along withthe circuitry 50 operates to first energize the alert 57 as a functionof the distance of the iron cover 30 from the coil 20. Therefore, thethinner the asphalt layer 31 the farther the spots 32 will be from thecenter of the cover 30.

The six spots 32 marked in the circular pattern shown in FIGS. 3 and 4are shown falling within the outer edge of the cover 30. It is notedthat since these spots 32 are all marked through the use of the sameconsistent method that all lie a constant distance from the center ofthe iron cover 30 and the center point of the cover 30 may be accuratelylocated. Thus, regardless of whether the spots 32 marked fall outside ofthe periphery of the cover 30, exactly along its outer boundary orwithin the outer edge of the cover 30, the exact location of the outeredge of the cover 30 relative to the asphalt 31 may be determined bydrawing a circle having the known radius of the standard sized cover 30about the center of the circle defined by the spots 32.

To remove the cover 30, the heater ring 33 is placed concentricallyaround the spots 32 of FIGS. 3 and 4, thereby to be positioned justoutside the outer edge of the cover 30 as shown in FIG. 4. It is notedthat the heater ring 33 also provides a pattern for convenientlylocating and delineating the outer edge of the cover 30 since it isdimensioned to be just slightly larger and of the same shape. Theasbestos blanket 40 is next placed over the heating element 33 and asuitable weight, such as, a chain, is placed on the asbestos blanketadjacent the element 33 to secure the blanket in place. The coil 20still connected to the circuitry 50 is then used to determine theapproximate depth of the asphalt layer over the cover 30 by placing thecoil 20 in the center of the element 33 and slowly raising it verticallyuntil the alert 57 is turned off by the operation of the circuitry 50.The vertical height of the coil 20 over the asphalt 31 is measured togive an estimated depth of the asphalt coating since the alert 57 isswitched off at a fixed distance from the cover 30 and this distanceequals the sum of the asphalt thickness and the vertical height of thecoil 20 over the asphalt 31. The heating element 33 is then energized byturning on the power supply 36 and the rheostat 37 is set to apply anappropriate voltage across the ends 34 and 35 of the heating element 33.

Depending upon the voltage applied to the heating element 33, whichdetermines the rate that heat energy is generated and the estimatedthickness of the layer of asphalt covering the cover 30, the heatingelement 33 is allowed to melt through the layer of asphalt for aselected period of time, usually from five to twenty minutes to detachthe asphalt adhering to the cover 30 from the rest of the asphalt layer31. The asbestos blanket 40 functions during this burning or meltingstep to prevent the undue loss of heat energy from the heating element33 and concentrates its application to the selected section of theasphalt 31. It is here noted that the step of determining the estimateddepth of the asphalt 31 by vertically raising the coil 20 until thealert 57 is switched off may be performed any time after a sufficientnumber of spots 32 are marked to determine the location of the portionof asphalt 31 situated over the center of the cover 30, such as, justprior to the positioning of the heating element 33 concentricallyrelative to the circle defined by the marks 32.

Once the asphalt layer 31 has been melted through to cut the asphalt 47on the cover 30 away from the rest of the asphalt layer 31 as is shownin FIG. by the cut 43 through the asphalt layer 31 around the edge ofthe cover 30, the heating ring 33 is removed and a pick 44 or otherimplement may be used to pry up and remove the cover 30. With themanhole cover 30 removed, the manhole may be checked for explosive gasand entered or inspected as desired. For example, if water is found inthe underground passageway associated with the manhole it will be pumpedout or if explosive or toxic gas is found therein, fresh air will beforced into the manhole. Also, with the manhole cover 30 removed, therim 41 of the manhole in which the cover 30 rests which is shown in FIG.5 may be cleaned, painted and a conventional riser ring may be installedtherein to raise the level of the manhole to that of the roadbed. Themanhole cover 30 is then heated on its underside, such as by a blowtorch, to soften the asphalt layer 42 thereon and detach it from thecover 30. The cover 30 is then replaced to close the manhole, and thecrack between the cover 30 and the rim structure 41 in which it ismounted is tilled, such as with asphalt, to seal the manhole. Theasphalt while hot may be tamped and smoothed to insure that a smoothsurface is provided around the manhole. With the manhole cover 30replaced, the area around the cover 30 is then cleaned to remove thedebris accumulated around the manhole during the above-describedprocedure of locating, removing and replacing the manhole cover 30.

In the foregoing there has been described a novel method for locating,removing and replacing a magnetizable object, or an object at leastportions of which can be magnetically detected; i.e., an iron manholecover in a roadbed, covered over by a layer of nonmagnetic substance,such as asphalt, with minimal damage to the surface of the non-magneticsubstance. Additionally, there has been provided novel apparatus fordetecting such objects which includes an inductive search coil to sensea magnetic object as a function of the amount of magnetic mass in theobject and its distance from the inductive coil. The inductive coilforms part of an oscillator circuit which decreases its ampli-' tude ofoscillations as the inductive coil increases in inductance due to theinfluence of the presence ofa magnetic object. Different shapes andsizes of inductive search coils may be used which are particularlydesigned for generally or accurately locating the magnetic object. It isnoted that exemplary circuitry 50 is arranged with an alert means whichis normally not activated but which activates upon the detection of amagnetic object. The circuitry, however, could be arranged so that thealert 57 is normally activated and is deactivated upon detection of amagnetic object.

It is therefore to be understood from the foregoing description of apreferred form of the present invention that various modifications andchanges may be made in the steps and order of steps in the method of thepresent invention and also in the specific design, construction andarrangement of circuitry and mechanism employed without departing fromthe spirit and scope of the present invention.

What is claimed is:

1. A method of accurately locating and removing a manhole cover ofpredetermined size and shape which is covered by a layer of asphaltcomprising the steps of:

sensing the general location of the manhole cover within the asphaltlayer;

magnetically detecting the outer peripheral edge of the cover andmarking on the asphalt layer the outline shape of the outer peripheraledge of the cover;

melting through the asphalt layer around the peripheral edge of thecover by covering the asphalt layer with an insulating material andapplying heat to the layer beneath the insulating material around theperipheral edge of the cover so as to separate the asphalt on the coverfrom the rest of the asphalt layer; and

removing the cover from the asphalt layer.

2. The method recited in claim 1 wherein the outer peripheral edge ofthe cover is magnetically detected by locating a plurality of marksrelative to the outer peripheral edge of the cover through the use of amagnetic detection apparatus, and wherein the magnetic detectionapparatus employed includes an inductive coil which changes inductanceas a function of the proximity and mass of the cover, oscillator meansincluding the coil which changes its amplitude of oscillation wheneverthe inductance of the coil increases due to the detection of the cover,and alert means operated by the oscillator means for indicating thedetection of the cover.

3. The method recited in claim 2 wherein the marks are located by movingthe inductive coil relative to the manhole cover so as to activate anddeactivate the alert means and the marks are marked relative to thepositioning of the coil in a consistent manner to establish the shape ofthe outer edge of the cover.

4. The method recited in claim 3 wherein the steps of magneticallydetecting the approximate depth of the asphalt layer over the coverincludes employing a heating element beneath the layer of insulatingmaterial to melt through the asphalt layer, and controlling the durationthe heating element is energized and the rate it generates heat energyaccording to the thickness of the asphalt layer.

5. The method recited in claim 4, wherein the depth of the asphalt layeris determined by moving the coil of the magnetic detection apparatusupward vertically from the portion of asphalt. situated over the locatedcenter of the cover to the position where the inductance of the coildecreases to the value at which the alert means is deactivated by theoscillator means, and the insulating material is an asbestos blanketwhich is secured over the heating element during its energization toprevent the loss of heat energy and direct the heat generated into theasphalt.

6. The method recited in claim 5 wherein the general location of themanhole cover is magnetically sensed.

7. The method recited in claim 6, including the step of replacing andresealing the removed manhole cover in place.

27 3 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. ,74,223 Dated 9 October 1973 Inventor(s) Frank J. Lucero, Jack A. Lucero,Gary' R. Weekly It is certified that error appears in theabove-identified patent and that said Letters Patent are herebycorrected as shown below:

In the Abstract:

Line 8, cancel "detectiom" and substitute detection In the TechnicalDisclosure:

Column 2, line 15, cancel "around" and substitute through Signed andsealed this 21st day 0:1 May 1971;.

(SEAL) Attest:

EDWARD l hFLl'lTfi IlElhJli. I C. IlARSHALL DAM-l Attesting OfficerCommissioner of Patents

1. A method of accurately locating and removing a manhole cover ofpredetermined size and shape which is covered by a layer of asphaltcomprising the steps of: sensing the general location of the manholecover within the asphalt layer; magnetically detecting the outerperipheral edge of the cover and marking on the asphalt layer theoutline shape of the outer peripheral edge of the cover; melting throughthe asphalt layer around the peripheral edge of the cover by coveringthe asphalt layer with an insulating material and applying heat to thelayer beneath the insulating material around the peripheral edge of thecover so as to separate the asphalt on the cover from the rest of theasphalt layer; and removing the cover from the asphalt layer.
 2. Themethod recited in claim 1 wherein the outer peripheral edge of the coveris magnetically detected by locating a plurality of marks relative tothe outer peripheral edge of the cover through the use of a magneticdetection apparatus, and wherein the magnetic detection apparatusemployed includes an inductive coil which changes inductance as afunction of the proximity and mass of the cover, oscillator meansincluding the coil which changes its amplitude of oscillation wheneverthe iNductance of the coil increases due to the detection of the cover,and alert means operated by the oscillator means for indicating thedetection of the cover.
 3. The method recited in claim 2 wherein themarks are located by moving the inductive coil relative to the manholecover so as to activate and deactivate the alert means and the marks aremarked relative to the positioning of the coil in a consistent manner toestablish the shape of the outer edge of the cover.
 4. The methodrecited in claim 3 wherein the steps of magnetically detecting theapproximate depth of the asphalt layer over the cover includes employinga heating element beneath the layer of insulating material to meltthrough the asphalt layer, and controlling the duration the heatingelement is energized and the rate it generates heat energy according tothe thickness of the asphalt layer.
 5. The method recited in claim 4,wherein the depth of the asphalt layer is determined by moving the coilof the magnetic detection apparatus upward vertically from the portionof asphalt situated over the located center of the cover to the positionwhere the inductance of the coil decreases to the value at which thealert means is deactivated by the oscillator means, and the insulatingmaterial is an asbestos blanket which is secured over the heatingelement during its energization to prevent the loss of heat energy anddirect the heat generated into the asphalt.
 6. The method recited inclaim 5 wherein the general location of the manhole cover ismagnetically sensed.
 7. The method recited in claim 6, including thestep of replacing and resealing the removed manhole cover in place.